Heat butter in a medium skillet over medium-high heat. Add mushrooms and sauté for 3-4 minutes. Add the onion and sauté another 3-4 minutes, until mushrooms are tender and slightly brown. Remove from heat and stir in parsley and cheese.

Heat the oil in a large nonstick skillet over medium-high heat. Add the chicken and season with salt and freshly ground pepper. Cook for about 5-6 minutes on each side, until the chicken is cooked through.

Serve the mushroom-onion mixture over the chicken.

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